From BR Bullpen
The National Agreement is a pact which governed relations between rival major leagues, allowing them to respect one another's player contracts and providing for a championship series between the two leagues' champion teams.
The first National Agreement was signed between the National League and the American Association in 1883. A third league, the Northwestern League, which is not usually considered a major league, was also a party to the agreement, which is sometimes called the "Tripartite Agreement" as a result. The Agreement ushered in an area of peaceful cooperation between the rival circuits and allowed for staging the first championship series between the champions of the two leagues, the 1884 World's Series. The Agreement broke down following the creation of the Players League by the Brotherhood of Professional Baseball Players, which led to an all-out war between the three major leagues. The three leagues came to a peace settlement after the 1890 season and created a National Board of Control to rule over disputes stemming from the return of players who had played in the no longer operating Players League, but the short-lived peace broke down when the Board ruled against the American Association's Philadelphia Athletics in the cases of Lou Bierbauer and Harry Stovey. The American Association formally denounced the National Agreement early in 1891, but disappeared after that season.
A second National Agreement was signed in 1904, ending the two-year war between the National League and the upstart American League. The two leagues created a three-member National Commission to settle disputes regarding player contracts and arrange for an annual post-season championship series, the World Series. The two leagues grew more integrated over the years, first with the creation of the office of Commissioner and the disbanding of the National Commission, as a result of the adoption of the Major League Agreement in 1920 in the wake of the Black Sox Scandal.
Other steps in the integration of the two leagues into a single entity were the negotiation of a single Collective Bargaining Agreement with the Players' Association, and eventually the suppression of the two positions of league President in 1999. The two leagues are now, for all intents and purposes, part of a single legal entity, Major League Baseball.
 Further Reading
- Charles C. Alexander: Turbulent Seasons: Baseball in 1890-1891, Southern Methodist University Press, Dallas, TX, 2011.